getFlake(): Use impureIsAllowed
getFlake(): Use impureIsAllowed

This fixes 'nix build nixpkgs:hello' without --impure.

NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix